A high voltage DC electrical power supply in a vehicle or piece of machinery presents a potential safety hazard to an individual when a leakage path exists between a terminal of the power supply and a frame of the vehicle or machinery. Typically, the vehicle or machinery frame will be electrically isolated from the terminals of the high voltage power supply. If an individual was to simultaneously touch one terminal of the power supply and the frame, the individual would be safe if the power supply remained isolated from the frame. As long as no electrical leakage path exists between the other terminal of the power supply and the frame, no current will flow through the individual. If an electrical leakage path does exist an unsafe amount of electrical current may flow through the individual.
Under normal conditions, only tiny leakage currents on the order of micro amps exist between the terminals of the high voltage power supply and the frame. In other words, the leakage resistance between the terminals of the high voltage power supply and the frame is normally very high. Because the leakage currents are so small, personnel safety is ensured. These small leakage currents may be due mainly to isolated base plates of switching devices in the high voltage DC electrical supply. Additional leakage paths may exist between the motor windings and the frame. In order to ensure personnel safety, it is necessary to check whether an unsafe leakage path has developed.
U.S. Pat. No. 5,382,946 to Gale discloses a circuit for detecting leakage resistance in an electric vehicle between a traction battery and the vehicle frame. The circuit operates by periodically applying an excitation signal from a low voltage excitation source to a capacitor for a selected time period. At the end of the time period, the voltage on the capacitor is measured and compared to a reference voltage. The circuit is configured such that the capacitor charges more slowly if a leakage path exists between the traction battery and the vehicle frame. By measuring the voltage on the capacitor after the selected time period, it can be determined whether a leakage path exists. If the traction battery voltage fluctuates significantly, however, such fluctuation may affect the rate of the charging of the capacitor and thereby cause false output indications.
The present invention provides a leakage detection circuit that avoids some or all of the aforesaid shortcomings in the prior art.